The present application generally relates to ship propulsion systems.
Propulsion devices for ship propellers with an electric propeller motor use rotation speed regulators for closed-loop control. A rotation speed nominal value is preset using the control lever on the bridge. Upstream of the input to the regulator, the rotation speed nominal value (reference variable) is compared with the rotation speed value at that time in order to determine from this a control error, which is supplied to the regulator. The output signal from the regulator is passed as a controlled variable to an actuating device, via which the propeller motor is connected to the current source.
When synchronous machines are used for propulsion, the actuating device is in the form of a frequency changer/converter, which uses the generator voltage from the diesel generator system to produce a suitable polyphase, variable frequency supply voltage. The converter circuit is designed such that the interconnection of the converter and synchronous machine results in a similar response to that from a DC machine whose current is controlled via a DC controller. The signal which is passed to the control input of the DC controller governs the current drawn by the DC machine. In the same way, the control signal from the regulator governs the current used to operate the synchronous machine. Asynchronous machines can also be supplied with electrical power, and can be used for ship propulsion, in the same way. It has now been found that propulsion systems of this type are relatively stiff, that is to say they are also able to regulate out minor rotation speed fluctuations which are within one propeller revolution.
The reason for rotation speed fluctuations and/or angular velocity changes is the behavior of the ship""s propeller in the water which is flowing past the hull while the ship is in motion and whose speed profile is not three-dimensionally uniform. During their rotational movement, the propeller blades in some places move through the skeg or propeller-shaft stay on the ship""s hull while, in the rest of their rotational movement, different water flow speeds impinge on them.
From the hydrodynamic point of view, the change in the load on the ship""s propeller with time can be described by its wake field. The fluctuation in this load which is caused by the skeg or propeller-shaft stay on the ship""s hull is once again evident in the inhomogeneity of the wake field of the propeller, which in turn results in a fluctuating angle of advance during revolution of the propeller blade.
Thus, the torque fluctuates cyclically, resulting in the ship""s propeller having a fluctuating angular velocity which is regulated out by the rotation speed regulator, or by the current regulator that is subordinate to it, in order to keep the rotation speed of the ship""s screw as exactly constant as possible at the preselected nominal rotation speed value. The frequency of the torque fluctuations corresponds to the shaft rotation speed multiplied by the number of blades on the propeller. The torque fluctuation is transmitted from the propulsion motor to its anchorage, and thus to the ship""s hull. A torque reaction also occurs on the diesel generator system. In consequence, parts of the ship structure are caused to oscillate at the fundamental frequency of this pulsating torque and, as a result of the mechanical characteristics, the resonance of the ship""s hull is not negligible at the relevant frequency. The vibration that this results in is not only annoying to those on the ship, but also results in a considerable load on the entire structure of the ship and its cargo, and should thus be avoided
In the past, attempts have been made to calculate the weak points for such oscillations using the so-called finite element method and to reinforce the critical areas determined in this way by the use of tons of steel. This method is on the one hand expensive and on the other hand reduces the maximum permissible cargo weight and the useful cargo area of the ship, while increasing the fuel consumption and, furthermore, although it can reduce those effects of the oscillations produced by the propulsion that destroy material, it does not eliminate the cause, however.
Closed-loop rotation speed control, which keeps the rotation speed of the ship""s propeller at the preselected nominal rotation speed value as exactly as possible, leads to a further negative effect.
Since the inhomogeneity of the wake field fully reflects the fluctuation in the angle of advance of the propeller, the cavitation safety margin of the propeller is reduced, since the operating point of a propeller becomes closer to its cavitation limit, or may even exceed it. Particularly in the region of a skeg or propeller-shaft stay on the ship""s hull, the operating point of the propeller may reach or exceed the cavitation limit and thus initiate cavitation, which can then lead to considerable damage to the ship and, in particular, to the propeller. Cavitation also leads to unacceptable pressure fluctuations and noise, which considerably reduce, in particular, the useful value and comfort of passenger, research and naval ships.
The rotation speed of ship""s propellers which are driven via electric motors can be adjusted very quickly. Rapid adjustment of the rotation speed also leads, inter alia, to cavitation on the propeller blades. In this case, the rate at which the rotation speed is adjusted depends on the speed of motion of the ship, that is to say on the incidence speed at which the water strikes the propeller.
For this reason, the ramp-up transmitters are provided, which, from the control engineering point of view, are located between the control lever and the nominal value input to the regulator.
When the actual rotation speeds of the ship""s propeller increase, its dynamic response changes considerably. Since the family of curves for the propeller (transition from the towing curve to the free drive curve) follow a square law, the maximum permissible dynamic response of the ship""s propeller decreases more than proportionally as the actual rotation speeds rise.
In the case of ship""s propeller propulsion devices which are known from the prior art, the ramp-up time which is governed by the ramp-up transmitter is increased in one to three stages as the rotation speed of the propulsion motor for the propeller increases, in order to keep the excess rotation speed within the maximum permissible range of the propeller curve.
Furthermore, with regard to the power requirement, the electrical propulsion system also has to take account of the generator excitation. Its time response is slower than the possible dynamic response of the electrical machine for the ship""s propeller.
Taking account of these two boundary conditions, the ramp-up transmitter from the prior art is designed as follows:
Starting from a rotation speed of zero, the propeller motor first of all accelerates without any restriction, that is to say optimally. The power consumed by the propeller rises more quickly while ramping up with a constant ramp-up time, and finally reaches a current limit in the rotation speed regulator, in order to avoid overloading the diesel generator system. At the end of the first stage of the ramp-up transmitter, a change is made to a different ramp-up time. The acceleration power which is available from the electrical propulsion decreases to virtually zero. This results in a sudden change in the power consumption from the diesel generator system, which it must, but cannot necessarily, regulate out. This leads to frequency and/or voltage fluctuations in the on-board power supply network.
At least in the first phase of the ramp-up time, the propulsion device draws electrical power from the diesel generator system, which in some circumstances leads to failure of the supply to the rest of the on-board power supply network.
When changing from the first ramp-up phase to the second ramp-up phase for acceleration of the ship, this results in the disadvantage that the ship is accelerated to only a very minor extent in certain rotation speed ranges.
With the propulsion device as described above, the current limit for the electrical machine for the propeller occurs at approximately 30% of the rated torque over the respective ship""s propeller curve. The region between the current upper limit of the electrical propulsion machine and the calculated ship""s propeller curve is required in order to provide a margin for heavy seas and/or ship maneuvers in addition to the acceleration torques which are required for the procedures involved in accelerating the ship.
The ramp-up transmitters which until now have been controlled in stages for propulsion devices for ship propellers are unable to allow the electrical machine which is driving the propeller to produce a defined acceleration torque during acceleration processes. In fact, over wide rotation speed ranges, they allow only the respective current limit at that time. The reason for this is that the acceleration time for the ship is several times the ramp-up time of the ramp-up transmitter type.
As has already been mentioned above, the diesel generator system has a power response with respect to time which can vary only more slowly than the power consumption of the electrical machine for the ship""s propeller. Thus, in addition to the restrictions resulting from the propeller curve, it is also necessary to take account of the restrictions which result from the maximum dynamic response of the generator system.
When designing diesel engines for diesel generator systems for ships, the requirements of the International Association of Classification Societies (IACS) are taken into account with regard to the load response. The three-stage load change diagram associated with these requirements has a considerable influence on the dynamic response of the propulsion device for the ship""s propeller in the case of present-day diesel engines, which use high boost levels. A further exacerbating factor is that the values that are known there are often no longer achievable nowadays, particularly in the upper power range, owing to inadequate maintenance and owing to the use of relatively poor quality bunker oil. The maximum possible dynamic response for power emission on the shaft of the diesel engine therefore, based on experience, decreases when the ship has been at sea for a lengthy time.
A further time gradient in the power emission from diesel engines, which is not specified according to the IACS or in any other generally binding form, is the thermal load capacity of the diesel engine. A smooth load change on a diesel engine at its operating temperature, from zero to the rated power or from the rated power to zero, may be carried out only within a minimum time, which is dependent on the physical size of the respective diesel engine. These times have fluctuated severely as a function of the physical size.
The time profile must not be exceeded, even in places, since, otherwise, this can lead to damage to the diesel engine.
The minimum times mentioned above may be between 10 and 20 seconds for small diesel engines, and up to 120 seconds for large diesel engines.
The converters/frequency changers which are connected between the diesel generator system and the electrical machine for the ship""s propeller require a control wattless component. The control wattless component is dependent on the load. Examples of converters/frequency changers such as these include current intermediate circuit converters, direct converters, converters for DC machines and the like.
The wattless component is supplied from the synchronous generators in the diesel generator system. The time gradient of the load-dependent wattless component for the converters mentioned above with a control wattless component may vary 15 to 25 times more quickly than the terminal voltage of the synchronous generators, and the generator system cannot follow this. In particular, time is required to educe the excitation field for the synchronous generators.
If the dynamic limits of the diesel engines are exceeded when driving ship propellers, their rotation speed fluctuates, and hence the frequency of the on-board power supply network that is fed from the diesel generator system, to an unacceptable extent. It is also impossible to preclude damage to the diesel engines when the closed-loop rotation speed control for the generator system is intended to, or must, keep the frequency of the on-board power supply network within a permissible range, while ignoring the dynamic limits. If the dynamic limits of the synchronous generators are exceeded, the voltage of the on-board power supply network also fluctuates so severely that it departs from the permissible tolerance band.
According to the prior art, experiments have already been carried out based on multistage or continuous changes to the ramp-up times of the rotation speed nominal value and/or the current nominal value in the course of trial runs for such a long time that it has been possible to regard the interaction between the electrical machine for the ship""s propeller and the diesel generator system as being satisfactory, without any unacceptable frequency or voltage fluctuations occurring in the on-board power supply network. In this case, it was often possible only to achieve optimization at certain operating points. There was no fixed relationship between the adjustment capabilities in the closed-loop control for the electrical machine for the ship""s propeller and its dynamic effect on the diesel generator system in the on-board power supply network. The time profile for the reduction in the load on the diesel generator system was rarely taken into account, and was rarely adjustable, in the closed-loop control for the propulsion device for the ship""s propeller.
Against this background, an object of an embodiment of the invention is to provide a ship propulsion system for a ship which has an electrical on-board power supply network. Preferably, one is provided which does not lead to reductions in comfort and/or to adverse effects on ship operation.
In particular, one aim is to make it possible to match, and to match the dynamic response of the ship propulsion system to the various types of boundary conditions mentioned above in a better manner.
According to an embodiment of the invention, an object may be achieved by developing a ship propulsion system. The reductions in comfort may be expressed in the form of oscillations in the ship""s structure and/or in flickering lighting. The device according to an embodiment of the invention ensures that no fluctuations occur in the instantaneous value of the on-board power supply network voltage and/or in its frequency, going beyond a reasonable extent, irrespective of the speed at which the control lever and/or the rudder angle is adjusted.
Fluctuations could thus occur in the on-board power supply network voltage if the control lever were reset to zero too quickly, with the load being removed from the generator system more quickly than is possible to reduce the excitation of the synchronous machine. Conversely, fluctuations can also occur if the control lever is moved too quickly in the direction of high motor power. As a rule, the frequency falls in this case, because the diesel engine cannot accelerate sufficiently quickly.
Rudder movements have a similar effect on the generator system and/or the on-board power supply network. As the rudder is deflected, the load on the propeller rises, while the load on the propeller decreases when the rudder is moved to the null position.
Excessively rapid acceleration processes on the propeller can also lead to considerable noise, if the acceleration leads to cavitation on the ship""s propeller.
The coupling of noise from the ship""s hull and from the propeller into the water represents environmental pollution which propagates over wide areas and can considerably restrict the use of ships in corresponding protected areas, for example in the Arctic and Antarctic. The reduction in the noise emission as described above makes it possible, in particular, for passenger ships to be operated in traveling regions which are financially of particular interest and in which the fauna living there remain protected against dangerous noise and pressure fluctuations, by virtue of an embodiment of this invention.
In order to counteract vibration which is produced because the ship""s propeller is subject to torque fluctuations in the moving water, the filters may include first filters which are set up to suppress amplitude fluctuations in the signal at the control input on the actuating device. Torque fluctuations result in changes to the angular velocity of the propeller shaft, which leads to corresponding ripple on the signal supplied from the rotation speed transmitter. Without an embodiment of the invention, the ripple would be reflected directly in the control difference and would lead to the current for the propeller motor, and hence its drive torque, fluctuating in accordance with this control difference.
The first filters filter out this ripple, that is to say the propulsion system may be provided with the capability to allow the rotation speed to flex when the propeller blades run into a high flow resistance, and allow the rotation speed to be resumed once the xe2x80x9cdifficulty impeding movementxe2x80x9d has disappeared.
The filters which can be used for this purpose may be amplitude filters which pass on a signal change only when the signal change has exceeded a certain level. A filter such as this may be, for example, in the form of a diode characteristic. The other option is to use a frequency filter which acts as a low-pass filter and filters out the ripple that is superimposed on the control difference.
The frequency filter may be designed to be adaptive in such a way that the cut-off frequency varies with the rotation speed of the propeller shaft, or the voltage threshold varies with the basic value or equivalent value of the input variable. This ensures that an adequate dynamic response is provided in all rotation speed ranges, without the suppression of the ripple having any influence on the closed-loop control dynamic response, or the ripple penetrating through to the actuating device in another rotation speed range.
The first filter may be arranged between the regulator input and the rotation speed sensor, in the signal path of the signal with the control difference, or at the output of the regulator between the regulator and the control input of the actuating device. It is also possible for the filter to be implemented in the actuating device.
If the filters are in the form of an amplitude filter, they are expediently located in the signal path for the control difference. The closed-loop control device preferably has a PI control response.
The closed-loop control device may be designed in a classic manner as an analog closed-loop control device, or such that it operates digitally.
In the case of a PI regulator, the desired filter characteristic is achieved by feeding back the output signal from the closed-loop control device in antiphase to the input. The actuating device for the propeller motor may itself once again be in the form of a regulator. The control signal for the actuating device in this case preferably has the significance of a current nominal value. That is to say, the current controlled may be that emitted from the actuating device to the propeller motor, hence controlling the torque which is emitted by the propeller motor. Such open-loop control is also possible when the propeller motor is in the form of a synchronous machine and the actuating device is in the form of a frequency changer or converter. Circuits that are suitable for this purpose are known from the prior art.
If feedback is used in order to filter the ripple, this feedback is expediently set such that it results in a steady-state control error of approximately 0.2 to approximately 3% at the rated load. If this control error has a disturbing effect, it can be compensated for by means of an appropriately corrected nominal value. The nominal value compensation may be carried out as a function of the estimated load.
In order to suppress cavitation phenomena on the ship""s propeller as a result of excessively fast acceleration, the filters expediently have second filters, which are in the form of controlled ramp-up transmitters. The ramp-up transmitter is used to match the rate of change of the rotation speed of the propeller shaft to the maximum permissible level.
For this purpose, the second filters may contain a characteristic in order that the rate of rise of the nominal value signal arriving from the control lever can be slowed down as a function of the rotation speed of the propeller motor. For this purpose, the second filters may be arranged between the input of the closed-loop control device and the control lever. At this point, it has no adverse effect on the control response, comprising a closed-loop control device, the actuating device and the ship""s propeller.
The characteristic of the second filter may be considered continuous in the sense that it has no discontinuities. It does not necessarily need to be smooth in the mathematical sense, but may also be approximated in the form of a string of polygons. The only essential feature is that the transitions within the string of polygons have no discontinuities. The characteristic may be a square-law characteristic with an offset.
In order that the ship can still be maneuvered well in the low speed range, the characteristic may be designed, at least in the lower rotation speed range, such that the ramp-up time is constant and short, and rises only slightly with the rotation speed of the propeller. The propulsion system is then effectively xe2x80x9cattachedxe2x80x9d directly to the control lever.
In a higher rotation speed range which starts at approximately 25 to 45% of the rated rotation speed, the ramp-up time increases with, or faster than, the rotation speed of the propeller motor. In consequence, the possible angular acceleration decreases the higher the rotation speed of the ship""s propeller, irrespective of the rate at which the control lever is moved.
In an upper rotation speed range which starts, by way of example, at half the rated rotation speed, the rate at which the rotation speed of the propeller motor can increase is restricted even further, that is to say the ramp-up time increases even faster with the rotation speed, than in the rotation speed range below this.
However, it would also be feasible to control the rotation speed of the propeller motor such that it rises firstly in accordance with a square law with a short ramp-up time and then with an increase in rotation speed of the propeller motor, in order that the rate at which the rotation speed of the propeller can increase is slowed down in accordance with a square-root function plus an offset.
The second filter may be in digital form using a microprocessor, or may be designed such that it operates in analog form.
As already mentioned in the introduction, reductions in comfort also occur when the on-board power supply network voltage fluctuates too severely, because the generator system cannot follow the change in the power requirement for the ship propulsion sufficiently quickly. Excitation of synchronous machines and, in particular, reduction in the excitation of synchronous machines, require time. If the power consumption by the ship propulsion changes more quickly than it is possible for the excitation/reduction in excitation to take place, the on-board voltage departs from the permissible tolerance band, and this unnecessarily loads, or overloads, the appliances which are connected to the on-board power supply network. The diesel drive for the generators cannot follow this sufficiently quickly either, and this can lead to damage to the diesel engine.
In order to eliminate adverse effects resulting from this, the filters may have third filters, which restrict the rate of change of the power consumption by the propeller motor, to be precise to values which the on-board power supply network system can follow without any problems.
The third filter may once again be arranged either in the signal path of the nominal value signal, that is to say between the regulator and the control lever, downstream of the closed-loop control device or in the actuating device itself. The arrangement downstream from the regulator or downstream from the subtraction point has the advantage of also slowing down state changes which are caused by changes in the propeller load. Such changes in the propeller load occur when moving the rudder or when switching off or throttling down a propeller in multishaft systems.
The third filters have expediently been embodied in digital form, based on microprocessors. The third filters may also be of classic design, and may operate in analog form.
The third filters may be designed such that they limit the rate of change, when the control lever is moved in the direction of greater power consumption, to values which are different to those used when the control lever is moved in the direction of low power values.
The limit to the rate of change decreases at least in an upper power range or rotation speed range of the propeller motor.
The rate of change which the third filters allow may also be dependent on the number of generators feeding the on-board power supply network. A further influencing variable may be the operating state of the system, that is to say whether the system is already in a warmed-up steady state or is still in the warming-up phase, that is to say it is dependent on the total operating time. Finally, a further influencing variable is the load on the generator system, that is to say whether the load is in the lower, the medium or the upper power range of the diesel engines.
In order that the ship remains maneuverable and, in addition, that no control oscillations occur which are caused by the limiting of the rate of change, the third filters may be designed such that they provide a window within which the third filters do not influence the rate of change at which the signal of the control input of the actuating device changes. A window such as this is particularly expedient when the third filters are located in the signal path between the closed-loop control device and the actuating device. If the third filters are located between the control lever and the nominal value input of the closed-loop control device, such a window is in some circumstances unnecessary. Those combinations of features which are not reflected by an exemplary embodiment are also intended to be covered by the scope of protection.
Where the patent claims refer to a xe2x80x9cship""s propellerxe2x80x9d and a xe2x80x9cpropeller motorxe2x80x9d, then it is obvious to those skilled in the art that the invention is not restricted to a single motor and a single ship""s propeller, but that a number of motors or ship""s propellers may also be controlled jointly or separately from one another. Furthermore, the invention relates equally to surface vessels and underwater vessels.